Bioluminescence and second harmonic generation imaging reveal dynamic changes in the inflammatory and collagen landscape in early osteoarthritis

Abstract

Osteoarthritis (OA) is a leading cause of chronic disability whose mechanism of pathogenesis is largely elusive. Local inflammation is thought to play a key role in OA progression, especially in injury-associated OA. While multiple inflammatory cytokines are detected, the timing and extent of overall inflammatory activities in early OA and the manner by which joint inflammation correlates with cartilage structural damage are still unclear. We induced OA via destabilization of the medial meniscus (DMM) in NFκB luciferase reporter mice, whose bioluminescent signal reflects the activity of NFκB, a central mediator of inflammation. Bioluminescence imaging data showed that DMM and sham control joints had a similar surge of inflammation at 1-week post-surgery, but the DMM joint exhibited a delay in resolution of inflammation in subsequent weeks. A similar trend was observed with synovitis, which we found to be mainly driven by synovial cell density and inflammatory infiltration rather than synovial lining thickness. Interestingly, an association between synovitis and collagen structural damage was observed in early OA. Using Second Harmonic Generation (SHG) imaging, we analyzed collagen fiber organization in articular cartilage. Zonal differences in collagen fiber thickness and organization were observed as soon as OA initiated after DMM surgery, and persisted over time. Even at 1-week post-surgery, the DMM joint showed a decrease in collagen fiber thickness in the deep zone and an increase in collagen fiber disorganization in the superficial zone. Since we were able detect and quantify collagen structural changes very early in OA development by SHG imaging, we concluded that SHG imaging is a highly sensitive tool to evaluate pathological changes in OA. In summary, this study uncovered a dynamic profile of inflammation and joint cartilage damage during OA initiation and development, providing novel insights into OA pathology.

NF-κB-mediated effects on behavior and cartilage pathology in a non-invasive loading model of post-traumatic osteoarthritis

“…Our group has previously shown increased but transient, joint specific, NF-κB activation following intra-articular injection of mono-iodoacetate to induce inflammatory arthritis, while other models have led to prolonged elevation of NF-κB activation 19 . In particular, NF-κB-driven luminescence has been reported to remain elevated beyond 6 weeks following surgically induced destabilization of the medial meniscus in a similar reporter mouse line 40 . This suggests the systemic effects of surgery, with consequences for prolonged elevation of inflammatory mediators, may elicit sustained alterations in joint inflammation and associated pain-related sensitivity and behavioral changes.…”

Transection of the medial meniscus anterior horn results in cartilage degeneration and meniscus remodeling in a large animal model

“…Most notably, destabilization of the medial meniscus (DMM) reliably induces OA in mice within 4 weeks 15 . DMM has been used to characterize the progression of post‐traumatic osteoarthritis (PTOA) functionally, structurally, biochemically, and biologically, and to assess the validity of various biological and surgical interventions in small animal models 15‐19 . The mouse knee joint is significantly smaller than the human joint, however, which limits the scope of this translational model.…”

Collagen Organization Within the Cartilage of Trpv4−/− Mice Studied with Two‐Photon Microscopy and Polarized Second Harmonic Generation

“…In order to determine whether cartilage is malformed when TRPV4 is absent during development, our study was conducted using a label‐free imaging technique based on the measurement of the SHG signal under two‐photon excitation. The structure of the osteoarthritic cartilage has been investigated before using SHG, but data were obtained on cartilage samples subjected to mechanical load, either after surgical induction of OA in mice or from patients undergoing knee surgery for OA . Our present contribution aims to correlate the structural modifications of the cartilage matrix with molecular mechanisms affecting the chondrocytes (i.e., reduction of Ca 2+ transients due to the absence of the TRPV4 channels) at a time point post‐cartilage development but before the tissue has been subjected to ongoing mechanical loads, that is, in pups at P4–5.…”